1. Field of the Invention
The present invention relates to a halftoning method and apparatus for converting a multilevel input image, such as a continuous-tone color image, into a binary image, and also to a computer-readable recording medium in which a halftoning program is recorded.
2. Description of the Related Art
For reproducing a tone-graduation image (e.g., multilevel image and continuous-tone color image) on a printer that is dedicated to binary outputting, it is necessary to convert values of the multilevel image into binary values. The binary-type printer realizes reproduction of tone graduation by modification in terms of dot area or dot density. In the following description, the terms “binarizing” and “halftoning” are used each as a word having a meaning “converting a multilevel image into a binary image”.
Consequently, in such halftoning, it has been required to make moiré and artifacts, which does not appear in the original image (multilevel image), unconscious and also to reproduce the tone graduation of the original image with the best possible fidelity.
The conventional halftoning technology is exemplified by the following:
(1) Japanese Patent Laid-Open Publication No. HEI 6-070144 (Application No. HEI 5-142746) discloses an attempt to realize a moiré- or artifact-free binary image by carrying out the error diffusion method along “a random spatial filling two-dimensional curve” throughout the entire original image.
(2) Japanese Patent Laid-Open Publication No. HEI 6-006586 discloses an alternative attempt to realize a moiré- or artifact-free binary image by minimizing energy functions that are calculated from values of the original image (multilevel image) and those of a binary image, utilizing the “simulated annealing method”. As this method two technique are currently known; one technique is to directly converting values of the original image, and the other technique is to previously obtain 256-grade dither patterns prior to converting values of the original image.
However, these conventional technologies would encounter the following problems:
(1) As to the technology disclosed in Japanese Patent Laid-Open Publication No. HEI 6-070144, because an entire image is processed by the same algorithm, moiré or artifacts would be inevitable with the converted binary image. In a patterned image such as a checkered one, in particular, horizontal bands tend to appear significantly apparently after halftoning. Under the method of “the random spatial filling two-dimensional curve”, since scanning is performed, during conversion, in a single continuous stroke-like manner, it is impossible to take parallel calculation, thereby requiring a huge processing time.
(2) As to the technology disclosed in Japanese Patent Laid-Open Publication No. HEI 6-006586, because huge time is needed in calculation during conversion, this technique would be less useful. As another disadvantage, in reduction of a very gentle tone-gradation image, a peculiar artifact would occur.
(3) In the above-mentioned conventional technologies, the multilevel pixel value of an object pixel (noteworthy pixel) to be halftoned is converted into a binary value, and an error accompanying the conversion is diffused to other pixels adjacent to the noteworthy pixel. So, in such technologies, if the pixel value of the noteworthy pixel is outstanding due to an image defect, a significant error risen from the outstanding value is also diffused to unprocessed adjacent pixels. For example, the moiré that is latently contained but not visually apparent in the original image, which moiré is due to periodic structure typical of an input device (scanner, digital camera, etc), could be enhanced by the error diffusing so that it becomes visually apparent after halftoned.